Process for oxidizing boron compounds with amine-n-oxides

ABSTRACT

PROCESS OF OXIDIZING ORGANO BORANE COMPOUNDS CONTAINING BC-, BH- OR BN-LINKAGES BY REACTING SUCH COMPOUNDS WITH SECONDARY OR TERTIARY AMINE OXIDES OR HYDRATES THEREOF AT TEMPERATURES FROM ABOUT ROOM TEMPERATURE TO ABOUT 120*C.

United States Patent U.S. Cl. 260-490 13 Claims ABSTRACT OF THE DISCLOSURE Process of oxidizing organo borane compounds containing BC-, BH- or BN-linkages by reacting such compounds with secondary or tertiary amine oxides or hydrates thereof at temperatures from about room temperature to about 120 C.

This invention relates to a process for oxidizing boron compounds with amine-N-oxides.

Organic boron compounds have increased in importance during the last years as intermediates for the syntheses of organic compounds chiefly because of the ease of preparing them from boron-hydrogen compounds. Special stress is to be laid in this connection on the conversion of unsaturated hydrocarbons which leads through oxidation of the organoboranes obtained by hydroboronation to a great variety of hydroxy compounds. Oxidation of organoboranes is carried out in most cases with alkaline aqueous hydrogen peroxide, which gives by no means always satisfactory yields of the alcohols. As in case of the oxidation with atmospheric oxygen, secondary and subsequent reactions may interfere. Moreover, only incomplete oxidation of the organoboranes is possible with atmospheric oxygen in most cases.

It is an object of this invention to provide an oxidation process which is generally applicable to boron compounds and which gives not only quantitative yields of uniform products but proceeds also under extremely mild conditions. It has been found that linkages of boron to carbon, hydrogen and to nitrogen of various boron compounds can be oxidized absolutely smoothly with N-oxides of tertiary amines, e.g. trialkylamine-N-oxides, pyridine- N-oxides and secondary amines, e.g. piperidine-N-oxide.

A11 BC-linkages of tris-organoboranes react, for example, quantitatively with trialkylamine-N-oxides. When starting from trialkyl boranes, triaryl boranes, boron heterocycles, there are obtained among other compounds the corresponding organic esters of orthoboric acid, e.g.

e.g.: R=alkyl, cycloalkyl, aryl; R'=CH Of course, alkyl borates RB(OR) and R B(OR) can also be prepared from trialkyl boranes and trimethylamine-N-oxide with corresponding stoichiometrical amounts.

While anhydrous N-oxides of tertiary amines (e.g. trimethylamine-N-oxide) are particularly advantageous as oxidizing agents for the process of the invention, the hydrates of N-oxides of tertiary and secondary amines (e.g. triethylamine-N-oxide-dihydrate) may also be used. However, saponifications of the resultant boric acid esters will simultaneously occur in this case. Moreover, protonolyses of readily cleavable BC and BH linkages cannot be avoided in this case. However, as compared with the previously known methods of oxidizing organoboranes with, for example, hydrogen peroxide or perbenzoic acid, the N-oxides with proton-active groups such as the N-oxide hydrates and N-oxides of secondary amines represent already a definite improvement. Undesirable byproducts are formed in many cases with peroxiy compounds due to the cleavage of the 0-0 linkages. Therefore, a uniform course of the oxidation of aryl boranes with peroxy compounds is not possible at all.

On the other hand, quantitative yields of uniform oxi dation products of the organoboranes are furnished by the process of the invention when using anhydrous N-oxides of tertiary amines. Due to the absence of any H-acid groups, protonolyses are no longer possible with these materials so that the protonolytically readily cleavable compounds containing BC and BC linkages as well as saturated organoboranes containing readily hydrolyzable BC linkages (1,1-diboryl alkanes) can also be readily oxidized quantitatively.

Trimethylamine-N-oxide has been found to be particularly advantageous for the process of the invention. Gaseous trimethylamine escaping from the reaction mixtures is liberated during the oxidation. This can be readily reconverted in known manner with hydrogen peroxide into the N-oxide. Acidimetric determination of the amine liberated permits also continuous control of the oxidations in progress. The amount of total BO linkages formed can be readily determined in this manner.

The process of the invention can be carried out in the simplest manner imaginable. Since the BC-, BH- and BN- linkages react generally much more rapidly with the N-oxides other than other functional groups of the compounds to be oxidized (e.g. C=C double bonds), secondary reactions will not occur even when using an excess of oxidizing agent. The reactions can be carried out with trimethylamine-N-oxide dissolved in alcohols or particularly well with suspensions of the N-oxide in aliphatic or aromatic hydrocarbons and in ethers of any type. However, in many cases, it is also possible to dispense entirely with a diluent. For example, the oxidation of an organoborane proceeds particularly smoothly when adding trimethylamine-N-oxide in portions. Temperatures of 20 to C., particularly 20 to 60 C. are generally sufiicient to oxidize completely all of the BC-, BH- and BN- linkages of organoboranes. For example, oxidation of a trialkyl borane will occur at as low as below room temperature. However, it is entirely possible to use higher temperatures to bring the reaction more rapidly to completion. However, heating to temperatures in excess of 140 C. should be avoided because serious decomposition of the amine oxide will then be encountered.

Boron compounds which are capable of being oxidized in accordance with the invention include not only the trialkyl and triaryl boranes already mentioned and organic boron heterocycles of any type. It is rather possible to oxidize any other compound having EC-linkages with trivalent boron (e.g. alkyl (aryl) halogen boranes, alkyl (aryl) boric acid esters, alkylamino boranes). In addition, certain addition compounds. can also be oxidized under mild conditions. Examples of these include the salt-like dialkyl boryl bis-amine halides [R B (amine) X in addition to amine-dialkyl-chloroboranes (R BCl-amine) Thus, it is possible by means of the process of the in vention to convert any hydroboronation product of unsaturated hydrocarbons quantitatively under mild conditions into the corresponding boric acid esters and consequently into the corresponding alcohols. Thus, in combination with processes of uniformly proceeding hydroboronations of alkenes, alkadienes, alkynes and similar 3 compounds, the invention permits the preparation of hydroxy compounds of high purity from these unsaturated hydrocarbons in a substantially quantitative yield. Among the essential and also surprising characteristics of the oxidation process of the invention is the fact that boron hydrides are not formed from the BC linkages, which would result in a loss of oxidizable BC portions. Moreover, no isomerization whatever of the organoboranes takes place under the mild reaction conditions during the oxidation. For example, it is easily possible to obtain pure cis-myrtanol in a quantitative yield from fl-pinene through cis-myrtanyl borane.

B CH2 CH nai 2 5 Smooth oxidation of compounds having more than one boron atom on the same carbon atom is also possible with N-oxides. For example, 1,1- and 2,2-diboryl-alkanes give boric acid esters of the hydrates of aldehydes and ketones, respectively, from which the free carbonyl compounds may be obtained. Boric acid esters of ortho carboxylic acid esters the saponification of which leads to the free carboxylic acids are formed from 1,1,1-triboryl alkanes. Reaction of vinyl boranes with N-oxides proceeds smoothly to form the boric acid esters of the correspond ing enols which give ketones or aldehydes when hydrolyzed. These last-mentioned organo-boranes are smoothly preparable at all only by means of the oxidation process of the invention because all previously known oxidations (e.g. with hydrogen peroxide, peracids, oxygen) fail in such cases because of protonolysis of the BC-linkages.

BH-linkages of the organoboranes and of the alkylated and non-alkylated polyboranes (e.g. decaborane) are also oxidized with the N-oxides. Apart from the alkyl diboranes and the boron hydrides, these include also adducts thereof with amines (e.g. trialkylamine boranes). Since the BH groups are initially converted into BOH groups in the oxidation reaction, cleavage of hydrogen will also occur with BH radicals which are not yet oxidized.

BOB +11 2 However, with BH-containing aminoboranes, these further reactions will not always occur. For example, the BH groups of the bis(dialkylamino)boranes are converted into BOH radicals which do not split ofi hydrogen under the conditions of the oxidation with the BH radicals still present.

BN linkages can also be oxidized by the process of the invention. This oxidation proceeds particularly smoothly and quantitatively in case of certain aminoboranes. For example, bis-piperidinoborane is oxidized quantitatively with 3 moles of trimethylamine-N-oxide:

INK-LN 3; HOB/ON/ 2 s NR3 2 EXAMPLE 1 Oxidation of trialkylboranes Tripropyl borane.To 14 g. (0.1 mole) of tripropyl borane are slowly added 22.5 g. (0.3 mole) of trimethylamine-N-oxide at a temperature of between and 80 C. Trimethylamine in an amount of 0.3 mole escapes while the temperature rises. After the addition is completed, 15.8 g. (100%) of boric acid tripropyl ester are obtained. The table below illustrates the results obtained when oxidizing other trialkyl boranes.

Moles Moles BB Me NO, Me N 1 (0 R in BR; g./mole g. lmole obtained obtained Yield C4Hg 18. 2/0. 1 22. 5/0. 3 0. 3 0. 1 Quantitative. C H 46. 8/0. 2 45/0. 6 0. 6 0. 2 Do. CeHra. 40/0. 15 34/0. 45 0. 45 0. 15 Do. OaHn. 35/0. 1 22. 5/0. 3 0. 3 0. 1 D0. CuHzp 60/0. 1 22. 5/0. 3 0. 3 0. 1 D0. C 11 85.4/0. 1 22. 5/0.3 0.3 0.1 Do.

EXAMPLE 2 Oxidation of tricycloalkyl boranes Tricyclohexyl borane.A total of 22.5 g. (0.3 mole) of trimethylamine-N-oxide is added in portions to a solu- Moles Moles BB3, MESNO, NOI-Ia B(OR)3 Compound R in BB; gJmole g./m0le obtained obtained CyclopropyL. 13. 4/0. 1 22. 5/0. 3 0. 3 0. 1

Cyclooctyl 34. 4/0. 1 22. 5/0. 3 0.3 0. 1

Cyclododecyl. 51. 3/0. 1 22. 5/0. 3 0.3 0. 1

EXAMPLE 3 Oxidation of triallyl borane A solution of 13.4 g. (0.1 mole) of triallyl borane in 50 ml. diethyl ether is slowly added dropwise to a suspension of 23 g. (0.3 mole) of trimethylamine-N-oxide in ml. of diethyl ether. Trimethylamine escapes and the ether begins to boil. The solvent is removed by distillation immediately upon addition of the oxide to give 18 g. of boric acid triallyl ester (B.P. 76-77 C./l7 mm. Hg) in a quantitative yield. Transesterification with methanol gives allyl alcohol (B.P.=97 C.).

Oxidation of one BC linkage of trialkyl borane is also possible with pyridine-N-oxide (0.1 mole). The pyridine formed is then best separated after substitution of hexane for the ether as BF adduct which precipitates as a sparingly soluble compound when adding boron trifluoride etherate. Diallyl-allyl-oxyborane can be recovered in this manner.

EXAMPLE 4 Oxidation of tris-2-cyclohex-3'-en-1-yl-ethyl borane 33.8 grams (0.1 mole) of tricyclohexenyl-ethyl-borane in 50 ml. of hexane are added dropwise to a boiling suspension of 23 g. (0.3 mole) of trimethylamine-N-oxide and 50 ml. of hexane. In doing so, the liberated amine escapes immediately. Upon completion of the addition (about 5 minutes), the solvent is distilled off to give a residue of 38 g. of boric acid ester (quantitative yield) which by mixing with 20 ml. of absolute methanol and distilling of the azeotrope of trimethyl oxyborane/methanol together with excess alcohol are converted quantitatively into 2-cyclo-hex3-en-l'-yl-ethanol (B.P. 118 .C./20 mm. Hg; n =1.4832).

EXAMPLE l (-)-Citronello1 from tris(3,7-dimethyl-oct-6-en-1-yl)- borane 54 grams of optically active 2.6-dimethylocta-2,7-diene ([a] =+7.9'6) are hydroboronated with 14 g. (corresponding to 0.2 g.-atoms of B) of tetraethyl diborane at room temperature. The excess octadiene (30 g.) is distilled 01f under reduced pressure (12 mm. Hg). After addition of 11 g. of about 0.5 ml. of triethyl alane, 11 g. of triethyl borane are obtained as distillate. After dilution of the remaining tris(3,7-dimethyl-oct-6-en-l-yl) borane with 50 ml. of benzene, 15 g. (0.2 mole) of trimethylamine-N-oxide are added in portions (from a swingable leg pipe attached to the flask). Thereby, the temperature rises and the mixture starts boiling. While the oxide disappears, trimethylamine escapes continuously and is collected in dilute (bl/.10) acid. (Found, about 0.2 mole.) Removal of the benzene by distillation gives 26 g. of boric acid ester (quantitative yield). Transesterification with methanol gives 24 g. (quantitative) of fl(-)-citronellol (B.P. 120 C./16 mm. Hg;

EXAMPLE 6 cis-myrtanol from fl-pinene 168 grams (1.215 moles) of fl-pinene are dropped rather rapidly into a three-necked flask cooled with ice Water and containing 100 g. of tetraethyl-diborane-triethyl borane mixture (1.34% hydride H=0.67 mole tetraethyl diborane), which results in an increase in the internal temperature to 2530 C. After stirring for additional minutes, olefin can no longer be detected by IR spectroscopy. However, the mixture still contains boron hydride. At a bath temperature of maximal 60 C./l2 mm. Hg (finally 0.3 mm. Hg), a total of 70 g. (0.715 mole, 84 of triethyl borane in addition to some ethyl diborane are distilled off while the receiver is cooled to 80 C. The completely crystallized residue (tri-cismyrtanyl borane, M.P. 94 C.) is dissolved in 500 ml. of warm absolute hexane and added dropwise to a suspension of 280 g. of trimethylamine-N-oxide in 2 liters of hexane at room temperature. Thereby, the mixture starts boiling and amine escapes as a vigorous stream. An almost clear solution is obtained. After removal of small portions of unconsumed N-oxide by filtration and of the hexane by distillation, 200 ml. of absolute methanol are added. Brief boiling and removal of boric acid trimethyl ester and methanol by distillation gives 185 g. (quantitative) of cis-myrtanol.

EXAMPLE 7 3 p,4a-dihydroxy-17fl-benzoyloxy-5 u-androstane The hydroboronation product (3/3-hydroXy-17fl-benzoyloxy-androstene(4)) prepared from testosterone benzoate (4 g.) with sodium boron hydride in methanol is oxidized by adding about 1 g. (13 mmoles) of trimethylamine-N-oxide to the boiling methanolic solution. After brief stirring, an azeotrope comprising boric acid trimethyl ester and methanol and the excess solvent are distilled off to give a residue of 4 g. (quantitative) of pure diol.

EXAMPLE 8 Oxidation of tris-4-chlorobutyl-borane 28 .5 grams (0.1 mole) of tris-4-chlorobutylborane (B.P. 126128 C./10** mm. Hg) dissolved in 100 ml. of benzene are added dropwise to a suspension of 24 g. (about 0.3 mole) of trimethylamine-N-oxide in 50 ml. of absolute benzene at 80 C. Trimethylamine escapes immediately. Upon completion of the addition, the mixture is briefly boiled, freed from unreacted oxide by filtration and from the solvent by distillation under vacuum to give as the residue 32 g. (quantitative) of tris- 4-chloro-butyloxyborane. B.P. 160-165 C./1 mm. Hg.

6 EXAMPLE 9 Tribenzylborane and trimethylamine-N-oxide Example 8 is repeated except that 56.8 g. (0.2 mole) of tribenzyl borane and 45 g. (0.6 mole) of trimethylamine-N-oxide are used to give a quantitative yield of boric acid tribenzyl ester; B.P. 180 C./3 mm. Hg; n =1.467.

EXAMPLE 10 Oxidation of triphenyl borane A solution of 10.3 g. (42 mmoles) of triphenyl borane in 150 ml. of toluene is added dropwise to a boiling suspension of 9.6 g. (128 mmoles) of trimethylamine-N-oxide in 50 ml. of toluene. The amine liberated quantitatively is introduced in an inert gas stream into N-sulfuric acid and determined by titration. Removal of small amounts of unchanged N-oxide by filtration and removal of the toluene by distillation gives 12 g. (97%) of boric acid triphenyl ester (M.P. C.). After addition of 10 ml. of absolute methanol and brief boiling, boric acid trimethyl ester and methanol are distilled off. The crystallizing residue (10 g.) is pure phenol (M.P. 41 C.). Yield, substantially quantitative.

EXAMPLE l1 Trinaphthyloxyboranes As in case of triphenyl borane,. a total of 39.2 g. (0.1 mole) of tri-l-naphthyl borane are oxidized with 23 g. (0.3 mole) of trimethylamine-N-oxide. Transesterification of the boric acid-l-naphtyl ester (M.P. 108 C.) gives 42 g. of l-naphthol (97.5%) having a melting point of 95 C.

In an analogous manner, 2-naphthol (M.P. 122 C.) can be obtained in. an almost quantitative yield from tri- 2-naphthy1 borane via tri-Z-naphthyl-oxyborane (M.P. 120 0.).

EXAMPLE 12 Phenyl actaldehyde from 1,1-bis(diethylboryl)-2- phenylethane 24 grams (0.1 mole) of 1,1-bis(diethylboryl)-2-phenylethane which can be prepared from phenyl acetylene and tetraethyl diborane are dissolved in 50 ml. of benzene and added dropwise to a suspension of 45 g. (0.6 mole) trimethylamine-N-oxide in ml. of benzene. Amine escapes immediately. Processing in usual manner gives a residue from which phenyl acetaldehyde (B.P. 193-194 C.) is obtained in an about 90% yield by transesterification with methanol.

EXAMPLE 13 Oxidation of ethyl diborane 14 grams (0.1 mole) of tetraethyl diborane in 50 ml. of hexane are added dropwise to a suspension of 45 g. (0.6 mole) of trimethylamine-N-oxide. Thereby, amine and hydrogen are evolved immediately. After the addition and brief boiling, unchanged N-oxide (7 g.) is removed by filtration. The hexane is distilled off. The residue consists of 16 g. of substantially pure tetraethoxydiboron oxide which partially splits off boric acid triethyl ester on heating.

EXAMPLE 14 Oxidation of bis-piperidyl borane 45 grams (0.6 mole) of anhydrous trimethylamine-N- oxide are added in portions to 38 g. (0.2 mole) of bispiperidyl borane in 100 ml. of benzene at about 80 C. while stirring. Trimethylamine escapes immediately. After completion of the addition and brief boiling, the solvent is distilled off to give a crystalline residue from which piperidine-N-oxide (M.P. 40 C.) is obtained in a substantially quantitative yield by transesterification with methanol.

7 EXAMPLE 15 The potential applications and the quantitative course of the process of the invention are illustrated in the following table with reference to a number of typical examcontains more than 1 boron atom attached to the same carbon atom.

8. Process according to claim 1, wherein said; borane compounds containing BH-linkages are selected from the group consisting of boron hydrides, substitution products ples of boron compounds which have been oxidized with 5 thereof with or anic residues and adducts of boron h trimethylamine-N-oxide in boiling toluene and/ or xylenes drides with amiies y on an analytical scale. In all cases, the trimethylamine liberated by the boron compound was determined 'acidi- Process f cla1m charactenzed m that metrically. Deviations of the values found from the calj g compounds contammg B N'hnkages are ammobo' culated values are due to contamination of the organo- 10 a boranes which were liquid in most cases and not to inprqcess. accorlimg to (51mm charactenzed m that complte Oxidations the reaction is carried out in a llqllld reaction medium Calculated Calculated B equivpercent Bl alents equivalent B Found,

Compound Percent B considered percent B Trlpropyl borane 7. 73 3 7. 73 7. 65, 7. 63 Y xy1borane 4.15 3 4.15 4. 07, 4. 09 Cyelohexyl diethyl-borane 7. 11 3 7. 11. 7. 06,7. 05 Z-phenethyl diethyl-borane 6. 21 3 6. 21. 6. 24, 6. 26 Tribenzyl-borane 3. 82 3 3. 82 3. 60, 3. 61 l-naththyldiethyl-borane 5. l5 3 5. 5. 18, 5. 19 l-propyl-l-boramdane 6. 96 3 6.96 6. 87, 6. 84 l-propyl-l-boratetrali 6. 28 3 6. 28 6. 24, 6. 26 Triphenyl-borane 4. 46 3 4. 46 4. 35, 4. 36 1,2,3,4-tetraethyl-5-m diboraeyclopent-2-ene- 11. 4 3 11. 4 11. 26, 11.35 1-propyl-1,2-oxaborolane 9. 68 2 6. 45 6. 40, 6. 42 2,3,3-triethyl-4,5-dimethy 1,2-

oxaborolane(4) 6. 01 2 4. 00 3. 99, 3. 97 Boro1anyl-2aminoethanolate- 7. 67 2 5. 11 5. 11, 5. O7 Dipropylchloroborane 8.17 2 5. 41 5. 32, 5. Dibenzylehloroborane. 4. 73 2 3. 15 3.06, 2. 96 Diethylehloroborane 10.35 2 6.91 6. 82, 6. 81 Piperidinodiethyl-borane 7. 07 a 7. 07 7. 04, 7. 05 Piperidinodipropyl-borane 5. 97 3 5. 97 5. 96, 6. 08 Diethylarnino-3-rnethyl-borolane. 7. 07 3 7. 07 7. 18 Diethyl-boryldi-pyridinium chloride 4. 12 2 2. 75 2. 75, 2. 71 Dipiperidinoborane 6. 00 3 6. 00 6. 00, 5. 99 Tetrakis(diethylamin0)-diboron 10. 94 5 9. 11 9. 22, 9. 25

What is claimed is: selected from the group consisting of alcohols, aliphatic 1. Process for oxidizing; borane compounds which conhydrocarbons, aromatic hydrocarbons and ethers. tain at least one linkage selected from the group consist- 11. Process as claimed in claim 8, wherein said BH coning of BC-, BH-, BN- to form oxygenated products ta1ning;tr1valent-borane compounds are alkyl substituted which process comprises reacting said; borane compounds organo-boranes. with a member selected from the group consisting of N- 12. Process as claimed in claim 8, wherein said BH oxides of secondary amines, N-oxides of tertiary amines containing; trivalent-borane compounds are alkyl suband hydrates thereof at temperatures from about room stituted polyboranes. temperature up to about 120 C. 13. Process as claimed in cla1m 2, wherein said N-oxide 2. Process according to claim 1, wherein said N-oxide Of a tertlary amlne 1s trlmethylamine N-oxide. is of a tertiary amine.

3. Process according to claim .1, carried out at up to References Cited about 80 Koster, et 51.; Angew. Chem. 73(11 589-90, June 7,

4. Process accordlng to cla1m 1, wherein said; borane 1966, compounds are tris-organo-boranes.

5. Pgrocess accordilngt t3 glaim {11], wherein sald Eris-org HENRY JILES, Primary Examiner ganooranes are se ec e rom e group consrs ing 0 trialkyl boranes, and triaryl boranes. SHURKO Asslstant Examlner 6. Process according to claim 1, wherein said; borane compounds are hydroboronation products of aliphatically unsaturated hydrocarbons.

7. Process according to claim 1, wherein said; borane US. Cl. X.R.

5353 UNITED STATES PATIENT OFFICE CER'PIFICIHMB OF CORIUSCLION Patent No. 3558633 Dated January 26 1971 Irwen Cx) Roland Koster It is certifi ed that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2; line 6, "peroxiy" should be corrected to --peroxy- Column 2 line 33 delete "other" (first occurrence) Column 2 line 63 should be Q Column 4, line 32, "oxidation" should be -oxidations-- Column 4, second table Third item, first column of figures "5l.3/0.l" should be corrected to --5l.2/0.l--

Delete the semicolon and insert --trivalent-- as follows Column 7, line 38, after "oxidizing";

Col. 7, lines 41, 51 56 and 59, after "said"; Column 8, line 3 after "said";

Column 8, line 41, after "taining" and Column 8, line 44, after "containing."

Signed and sealed this 10th day of August 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SGHUYLER, J'E

Attesting Officer Commissioner of Patents 

